1. Field of the Invention
The present invention relates to an acceleration detecting device including a fixed electric contact and a movable electric contact capable of coming into contact with the fixed electric contact at an acceleration of more than a certain amount
2. Prior Art
This kind of acceleration detecting device is utilized in, e.g., a conveyance, such as a motor vehicle for actuating an air bag in the event of collision. For example, JP-A-9-211023 discloses this kind of acceleration detecting device. FIG. 7 is a sectional view showing a conventional acceleration detecting device disclosed in JP-A-9-211023. As shown in FIG. 7, the acceleration detecting device comprises a weight 1, a housing 2, movable electric contacts 3a and 3b, a coil spring 4, fixed electric contacts 5a and 5b, and a guide shaft 11. The housing 2 defines a hollow or cavity where a straight guide shaft 11 is disposed in such a manner that the longitudinal axis thereof is aligned with the center axis of the cavity. The weight 1 is slidably mounted on the guide shaft 11.
The housing 2 has end walls 12 and 13 at which the guide shaft 11 terminates. The guide shaft 11 passes through the coil spring 4 that is arranged between the left end wall 12 and weight 1. As indicated by the imaginary line in FIG. 7, the restituting force of the coil spring 4 presses the weight 1 into the right end wall 13 at an ordinary state wherein no or little force is applied to the weight 1. When a large acceleration is applied to an apparatus, such as an automobile into which the acceleration detecting device is incorporated, a powerful force that is greater than the resilient force of the coil spring 4 may be exerted leftward, so that the weight 1 travels leftward and finally enters the cavity of the housing 2 as indicated by the solid line in FIG. 7.
Fixed electric contacts 5a and 5b made of a conductor are fixedly secured to inner surfaces of the upper and lower walls of the housing 2, respectively. Movable electric contacts 3a and 3b manufactured of a conductor are mounted on the weight 1. While the weight 1 travels and stays within the cavity of the housing 2, the movable electric contacts 3a and 3b are in contact with the fixed electric contacts 5a and 5b, respectively. Although not apparently shown in the drawings, the movable electric contacts 3a and 3b are interconnected inside the weight 1. Accordingly, contacting between the electric contacts 3a and 5a and between the electric contacts 3b and 5b results in the creation of an electrical connection between the fixed electric contacts 5a and 5b. 
Next, operations of the acceleration detecting device will be described,
When a force greater than a certain level is exerted leftward, the weight 1 slides on the guide shaft 11 and enters the cavity of the housing 2. When the movable electric contacts 3a and 3b come into contact with the fixed electric contacts 5a and 5b, the fixed electric contacts 5a and 5b are connected electrically with each other, so that an electric power is provided to an end effector, for example, an air bag activator. In other words, it can be detected that an acceleration of more than a certain amount is applied.
It is preferred that the movable electric contacts are in continual contact with the fixed electric contacts during the travel of the weight 1 in the cavity of the housing 2. Therefore, the spring constant of the coil spring 4, the length of the housing 2 along its longitudinal direction, the length of the fixed electric contacts 5a and 5b, and the stroke of the weight 1 are determined on the basis of the maximum extent of expected acceleration or external force.
However, if the external force is greater than the calculated maximum, the weight 1 may collide against the left end wall 12 that is the stroke limit for the weight 1 in the housing 2. This may cause instability of the electrical connection between the electric contacts. More specifically, if the weight 1 collides with the end wall 12, the backlash causes the movable electric contacts 3a and 3b to oscillate or vibrate as indicated by arrow A and the oscillation or vibration may continue for a while. In addition, it is possible that the weight 1 rebounds from the end wall 12 and exits from the cavity of the housing 2 with the result that the electrical connection between the electric contacts is interrupted. In this event, although a great acceleration is exerted, the time period of electrical interconnection may be too short to drive the end effector.
To avoid this consequence, some acceleration detecting devices are provided with cushions made of rubber and the like. Such a cushion is located at the position where the weight and the housing are likely to collide with each other in order to dampen shocks and reduce the velocity of the weight, thereby ensuring a sufficient time of electrical interconnection when a collision of a vehicle occurs.
However, with reference to such a conventional cushion made of rubber and the like, the hardness or the modulus of elasticity of the cushion may vary according to temperature, so that the expected buffering effect is not guaranteed. Especially, temperature is remarkably changeable and inconstant in a vehicle such as an automobile.
Furthermore, in accordance with a conventional device, it is inevitable to conduct a careful designing of elements, such as lengthening the stroke of the weight in order to ensure a sufficient time of electrical interconnection that is necessary for guaranteeing safety in a vehicle.
Accordingly, it is an object of the present invention to provide an acceleration detecting device that is not significantly affected by an environmental temperature and can ensure a sufficient time of electrical connection when an acceleration greater than a certain amount is exerted.
In accordance with an aspect of the present invention, an acceleration detecting device comprises a movable weight, a movable electric contact disposed on the weight, a fixed electric contact, an resilient member, and a friction creating mechanism. The movable electric contact slides on the fixed electric contact when a force is given to the weight for moving the weight in a first direction. The resilient member urges the weight continually in a second direction opposing to the first direction. The friction creating mechanism gives a frictional force to the weight when the weight travels a distance in excess of a certain amount in the first direction. The friction creating mechanism increases the frictional force as the weight continues to advance further in the first direction.
In a preferred embodiment, the friction creating mechanism includes a fixed brake fixedly arranged in place and a movable brake disposed on the weight. The movable brake may come into contact with the fixed brake for initiating the application of a frictional force to the fixed brake when the weight travels a distance in excess of a certain amount in the first direction.
At least one of the fixed brake and the movable brake may be elastically deformable and may generate a frictional component derived from elastic deformation thereof as a component of the frictional force.
Preferably, the elastic deformation of at least one of the fixed brake and the movable brake may progress as the weight continues to advance further in the first direction after the movable brake comes into contact with the fixed brake.
The movable brake may be formed as an integral part of an element that incorporates the movable electric contact.
In another preferred embodiment, the friction creating mechanism may include a protrusion fixedly arranged in place, a leaf spring disposed on the weight, a fixed brake fixedly arranged in place, and a movable brake. The leaf spring comes into contact with the protrusion for commencing to deform elastically when the weight travels a distance in excess of a certain amount in the first direction. The movable brake is capable of being in contact with the fixed brake. The movable brake progresses the deformation and increases the frictional force to the fixed brake for the increase of elastic deformation of the leaf spring.
Preferably, a depression is formed on the weight for receiving the protrusion so as to enable the leaf spring to deform elastically after the leaf spring comes into contact with the protrusion.
Preferably, the leaf spring is formed as an integral part of an element that incorporates the movable brake.
The movable brake may be the movable electric contact while the fixed brake may be the fixed electric contact.